Modern radar systems, e.g., active electronically scanned array (AESA) radar systems, have aggressive speed and power requirements for switching on and off the distributed RF amplifier power to the radio frequency (RF) transmit and receive amplifiers. The faster the power switches and settles, the greater the resulting RF signal energy transmitted and received by the antenna and the faster the echo sensitivity. The more efficiently the power is switched on and off, the smaller and/or lighter the power system may be.
High power amplifiers (HPA) are controlled by HPA switches that operate at high voltages, e.g., greater than 24V. Operation at such high voltages involves the use of slower devices to switch on and off the distributed RF amplifier power. For example, a large, slow pass transistor may be used to switch distributed RF amplifier power. An active pull-down circuit is used to increase the speed of the high-to-low transitions for an HPA. Design challenges include the timing of the active pull-down circuit too fast or too slow may lead shorts at the supply voltage, e.g., often designated Vdd, or decreases switching speed and may damage the antenna.
Power switches have been used in the past for switching on and off distributed RF amplifier power to RF amplifiers and in power converters. However, these designs use several discrete components are thus larger, provide slower switching, use more power to operate, and/or have a limited operating voltage range. Active Phased Arrays use individual solid-state transmit/receive (T/R) microwave module elements at each of its radiating elements (antennas), thus avoiding the distribution and phase shifter losses encountered in the passive array design. The functions of a T/R module may include the generation of the transmit power, the low noise amplification of the received signals coupled to and received from the respective radiating element, the phase shift in the transmit and receive mode for beam steering, and the variable gain setting for aperture weighting during reception. T/R modules are switched from transmit to receive quickly. Thus, the transmit gain path may be turned off during receive, and the receive amplifier path may be biased off during transmit.
Power amplifiers may be turned on by activating a discrete drain modulator that controls the voltage and the direct current (DC) power at the drain of power field effect transistor (FET) switches. To provide highly efficient switching while minimizing wasted power, discrete drain modulators may utilize FET switches with low Rds(On). Rds(On) is the drain-source resistance at a specified drain current and gate-source voltage. Active pull-down circuits are used to quickly discharge output loads. The active pull-down circuit may be a FET or bipolar junction transistor (BJT) with a control signal delayed from the main switch FET control signal.
These approaches are board layout and temperature dependent that result in long control delay calibration, wherein uncertainty in the delay leads to a slow design. Driving a large power FET with fast transitions times consumes significant power to charge/discharge the gate and involves adding components to minimize power, which uses additional board area. Consumer devices, such as cell phone, do not have timelines as challenging as radar. Thus, consumer devices may use a linear regulator pass FET. The response time is slower than a dedicated drain modulator solution because a regulator needs to limit bandwidth for stability.
Discrete drain modulator designs have difficulty timing the active pull down circuit. Moreover, to provide adjustable control signal delay involves performing a component swap for each board design. High power FETs used for low Rds(On) are discrete parts with separate drive circuit. The switch drive uses high current for fast switching, but uses more power when idle. Commercial integrated circuit solutions are regulator based and therefore slow or have a high Rds(On).